Cylindrical linear actuator devices are well known. FIG. 1 provides a schematic cross section of an example variable airgap reluctance actuator 1. The actuator 1, in which the airgap gradually closes up, has an armature 2 attracted to a stator core 3. Such linear actuators are particularly suited to applications which require relatively high levels of force and a robust construction. In such circumstances, these actuators can be utilised for linear actuation situations within relatively hostile gas turbine environments such as with respect to active control of blade tip clearance, vibration cancellation and other miscellaneous situations where a linear motion is required.
As can be seen in FIG. 1 an electrical coil or coils 4 are provided within the stator core 3. In such circumstances when the coil or coils 4 are energised, relative movement in the direction of arrowheads 5 is provided in an antagonistic relationship with magnetic attraction causing movement in one direction and typically gravity or a return bias spring or other mechanical device which produces a force that opposes the actuator. It will also be understood in certain circumstances the direction of electrical current flow in the coils 4 may be switched in order to cause the relative movements. Thus, by the effects of the coils 4 and a return bias/gravity respective movements in the direction of arrowheads 5 is provided as required.
Although actuators of the type shown in FIG. 1 are capable of producing large specific forces with a displacement in the direction of arrowhead 5, the general construction of the actuator 1 has a disadvantage in that the magnitude of the reluctance force at a given current varies approximately with the square of airgap width between opposed surfaces 6, 7 dependent upon such effects as saturation. In such circumstances, application of variable airgap reluctance actuators is currently limited to displacement strokes which are normally, but not exclusively, in a range below 1 mm.
Clearly, there is a significant requirement for medium displacement actuators which can cause displacement in the range of a few millimetres, but in view of the structure as described above, provision of variable airgap reluctance actuators for such longer range displacement applications is impeded by the size and mass related penalties with regard to the size of the armature and stator core as well as electrical coils. FIG. 2 provides a graphic illustration of predicted force to displacement characteristics for three optimised reluctance actuator designs which are capable of producing 1 kN displacement forces for 1, 2 and 3 mm armature displacement strokes. It will be noted in each case the armature and stator core are manufactured from a mild steel, while the electrical current densities in the coils are set at 5 amps per sqm due to thermal considerations with a copper packing factor of 65%. In such circumstances, as can be seen, for a 1 mm displacement stroke a 2.09 Kg actuator is required, whilst for a 2 mm displacement stroke a 3.8 Kg actuator is required and a 3 mm displacement stroke results in an actuator with a mass of 5.7 Kg. In such circumstances, it will be understood that there is a considerable increase in the actuator mass associated with extending a 1 kN force capability to longer displacement strokes. Such limitations severely limit the convenient use of airgap reluctance actuators in severe environments, such as those associated with aerospace applications.